In the modern communications network space, signal reach and spectral density are important factors in overall network cost. Assuming other factors to be equal, increases in either signal reach or spectral density tend to reduce overall network cost and are thus very attractive to network service providers.
Signal reach is the distance that an optical signal can be transmitted through a fiber, before conversion to electronic form is required to perform signal regeneration. Using suitable optical amplifiers and optical processing techniques, between 10 and 20 fiber spans (of 40–80 km each) can be traversed by an optical signal before optical/electrical conversion and regeneration are required.
Spectral density, which is normally expressed in terms of bits/sec/Hz (b/s/Hz), is a measure of the extent to which the theoretical maximum bandwidth capacity of an optical channel is utilized. This value is generally determined by dividing the line rate (in bits/sec.) of a channel by the optical frequency (in Hz) of that channel. A spectral density of 1 indicates that, for a given channel, the line rate and optical frequency are equal. Existing telecommunications systems commonly operate at line rates of approximately 2.5 Gb/s to 40 Gb/s. At a line rate of 10 Gb/s, current Wavelength Division Multiplexed (WDM) (or Dense Wave Division Multiplexed (DWDM)) transmission systems achieve a spectral density of approximately 0.1 b/s/Hz. If the line rate is increased to 40 Gb/s, the spectral density increases to approximately 0.4 b/s/Hz, illustrating the advantages of increasing the line rate.
However, increasing the line rate raises a number of difficulties. In particular, at line rates of about 10 Gb/s and higher, the optical characteristics of a fiber link have an increasingly important effect on the signal-to-noise ratio at the receiving end of the link, which manifests itself as an increased bit error rate of the regenerated signal. In addition, the physical limitations of electronic circuits at each end of the link mean that signal processing becomes progressively less reliable as the line rate increases. Both of these difficulties become of particular importance at line rates of 40 Gb/s, and present significant obstacles to further increases in line rate beyond this speed.
One known method of addressing the problem of increased bit error rates at higher line rates is to encode the data signal using a known Forward Error Correction encoding scheme (e.g. BCH) prior to transmitting the signal through the link. The FEC-encoded signal can be decoded at the receiving end of the link, to recover the original data signal. FEC encoding enables errored bits within the recovered data signal to be corrected at the receiving end of the link, and thereby enables effective data transport in spite of the increased bit error rates encountered at high line rates.
Difficulties resulting from the physical limitations of electronic circuits can be addressed by dividing the signal into a plurality of parallel sub-streams, which can be parallel processed at a lower line rate (e.g. 2.5 Gb/s). The sub-streams can be independently FEC encoded, and interleaved (e.g. using a conventional sequential interleaving process) into a single high-speed signal having a line rate exceeding that which can be accommodated with conventional electronic signal processing hardware. However, the success of this approach is entirely dependent on the ability of the node at the receiving end of the link to successfully separate the signal to recover each of the sub-streams at the receiving end of the link. This, in turn, requires a framer capable of correctly identifying each frame of each sub-stream within the high-speed signal, which is at least partially dependent on the bit error rate of the high-speed signal. For example, within the Synchronous Optical Network (SONET) protocol, a framer can typically tolerate a bit error rate of less than about 0.01 without declaring a loss-of-frame condition.
However, as mentioned previously, the bit error rate of a received signal tends to increase with higher line rates. The need to keep bit error rates lower than about 0.01 imposes a significant limitation, which frequently requires a designer to trade off signal reach in order to attain higher line rate (spectral density).
One way of improving the operation of framer is to insert a predetermined unique synchronization word into each sub-stream prior to interleaving the sub-streams into the signal. The framer can then identify each of the sub-streams by searching the signal to detect each of the unique synchronization words. However, this arrangement suffers limitations in that correct detection of the unique synchronization word is affected by the bit error rate, and thus degrades as the bit error rate increases. Additionally, because the unique synchronization word is inserted into each sub-stream at regular periods (typically at each frame), the energy spectrum of the signal becomes characterized by a recurring bit pattern having a fixed frequency. This is detected within both optical and electronic signal processing devices as an harmonic signal component, which tends to degrade the efficiency of these signal processing devices.
Accordingly, a method and apparatus for reliably framing high bit error rate signals composed of multiple interleaved sub-streams, while maximizing the efficiency of signal processing equipment, remains highly desirable.